Digital magnetic compass

ABSTRACT

A magnetic compass in which the compass card and gimbal mounting are all located inside a sealed enclosure filled with damping fluid. The compass card is digitally coded and photoelectric means are provided for reading the card and generating a digital number corresponding to the rotational position of the card relative to the enclosure, corrected for local magnetic variation. Means are provided for comparing the compass reading with the corrected course reading to provide an error signal or an off-course alarm if desired.

United States Patent [191 Fowler July 17, 1973 DIGITAL MAGNETIC COMPASSPrimary ExaminerMaynard R. Wilbur F l W th M [75] Inventor John T ow er,m mp ass Assistant Examiner-Robert F. Gnuse [7 A gn The Lain-amCorporation, Ne Attorney-Robert J. Schiller et al.

Orleans, La.

[22] Filed: Sept. 27, .1971 [57] ABSTRACT [21] Appl. No.: 184,003 Amagnetic compass in which the compass card and gimbal mounting are alllocated inside a sealed enclo- [52] U 8 Cl 235/92 CV 235/92 R 235/92 CAsure filled with damping fluid. The compass card is digi- '66' EC340/347 235/150 tally coded and photoelectric means are provided for [511 Int Cl Go6m 1 1 reading the card and generating a digital numbercorre- 58] Fie'ld CA 92 CC sponding to the rotational position of thecard relative 235/92 d 92 to the enclosure, corrected for local magneticvariation. Means are provided for comparing the compass reading with thecorrected course reading to provide an [56] UNITE S ;T; E S :Z?rENTSerror signal or an off-course alarm if desired. 3,637,997 H1972 Petersen235/92 CA 10 Claims 5 Drawing l e 221:: JO

GEN.

76 i M 92 FIRST FF SECOND COMPARATOR COMPARATOR as 98 J |eo READ 82CLOCK osrecr. OUT so 7 I00] 94 o FIRST SECOND J COUNTER COUNTER 1 SW JMAG. I02 VAR. GEN.

' Patented Jt llyfl7, 1973 3 Sheets-Sheet, 1

' JOHN T. FOWLER A TTORNEYS INVENTOR.

Patented July 17, 1973 v 5 Sheefis-Sheet 2 DESIRED 9O COURSE 7 GEN.

92 FIRST FF SECOND J COMPARATOR COMPARATOR READ v CLOCK OUT DETECT. '80j FIRST SECOND J COUNTER COUNTER v SW lOl vAR.

GEN. INVENTOR.

JOHN T. FOWLER g) pana idcib A T TORNEYS Patented July 17, 19733,746,842

3 Sheeis-Sheet 3 T 2|9 I f 200'\ 20|-. :202 203 204 205 SLOW l r 206CLOCK GRAY TO BINARY J CONVERTER s R v FF 2l 6 2:2 3 i- T START STOPCOMPARATOR I FAST \ZM I CLOCK g 2|O BINARY RIPPLE COUNTER I BlNARY CODEDDECIMAL 2l8 I COUNTER l i l I I /'220 DISPLAY F/G.-5 l

INVEN'IOR.

JOHN T. FOWLER 9 pant/[5050 ATTORNEYS DIGITAL MAGNETIC COMPASS nets is arotor element such as a compass rose or the like, having graphic indiciathereon. In top-viewing compasses, the element is usually asubstantially flat card, while in front-viewing compasses the element isusually a drum. The rotor, magnets and medium are enclosed in a bowlhaving a window through which the graphic indicia can be viewed andcorrelated with a fiducial mark. Gimbal arrangements to keep the magnetshorizontal are usually independent of the compass rotor.

In order to ascertain the compass heading as shown by the relationshipbetween the indicia on the rotor and the fiducial mark, the usergenerally must be close to the compass. Hence, the standard type ofcompass is not suitable for remote reading. Additionally, correlationbetween the indicia and fiducial mark is often inexact due to parallaxproblems, and the close spacing of the degree indications, particularlyin compasses having a small diameter (e.g., 4 inches) rotor, oftencontribute to inaccuracy in reading the devce.

A principal object of the present invention is to provide a magneticcompass capable of providing an output in the form of A group of digitalsignals which can be displayed and read at remote points or otherwiseprocessed for display and/or control. Other objects of the presentinvention are to provide such a compass in which the directionalrelation between the compass mounting and the rotor is determinedphotoelectrically; to provide such a compass in which the directionalrelation is directly derived as a digital, as distinguished from analog,value; to provide such a compass which is adjusted for local magneticvariation and to provide such a compass including means for comparingthe compass heading with a desired courseso as to provide anioff-courseindication.

To achieve the foregoing and other objects, the invention generallycomprisesa rotor fixed to and rotatable with a magnetic body, a framesupporting the rotor for rotation of the latter, and an enclosure orbowl surrounding the frame and rotor. The frame js supported formovement within the bowl through two degrees of freedom by gimbal means,and the bowl is preferably filled with a fluid medium for damping motionof both the rotor and frame. The rotor bears thereon a coded mask andthe frame supports a light source disposed to one side of the mask andphotoelectric means disposed to the other side of the mask, all arrangedso that the position of the rotor with respect to the frame is uniquelyidentified by the nature of the light signals detected by thephotoelectric means. The light signals are employed to generate adigitally coded signal corresponding to the position of the rotor withrespect to the frame. Means are provided for converting the coded signalto a decimal course display, by comparing the coded signal with thecount in a clocked counter means and displaying the count in the countermeans reached when the comparison results in stopping the clock. In apreferred embodiment, the compass output is compared in a firstcomparator to the state of a first counter and a selected desired courseis compared in a second comparator to the state of a second counter,both counters counting the same clock. Following comparison, thedifference between the two counter states. is indicative of the extentto which the compass and desired course differ. Digital logic isprovided for adjusting the operation of the counters and clock toaccomodate for the counting problems arising out of the transitionbetween zero and 360.

Other objects of the invention will in part be obvious and will in partappear hereinafter.

The invention accordingly comprises the method and apparatus involvingthe several steps and the relation and order of one or more of suchsteps with respect to each of the others and the apparatus possessingthe construction, combination of elements and arrangement of parts whichare exemplified in the following detailed disclosure, and the scope ofthe application of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detaileddescription taken inconnection with the accompanying drawings wherein:

FIG. 1 is an elevational view partly in section of a compass embodyingthe invention, the section being taken through the axis of rotation ofthe compass rotor;

FIG. 2 is a fragment showing, in plan the rotor of the embodiment ofFIG. 1;

FIG. 3 is a perspective view showing the gimbal arrangement of thecompass of FIG. 1;

FIG. 4 is a block diagram showing an electrical system for adjusting anddisplaying the output of the compass of the present invention; and

FIG. 5 is a block diagram showing another electrical system of thepresent invention for use in adjusting and remote display of the compassoutput.

Referring now to the drawings there is shown in FIG. 1 one embodiment ofthe present invention including enclosure 20. The latter is formed of anon-magnetic (i.e., diamagnetic or paramagnetic) material preferably arigid organic polymer such as polyvinylchloride or the like.Enclosure'20 is shaped in a hollow, substantially spherical form madefor convenience, out of two hemispheres 21 and 22 with matingperipheral-flanges 23 and 24 which can be locked together by bolts 26 toform the desired spherical shape. In order to insure against fluidleakage between flanges 23 and 24, the latter are grooved to form, whenlocked together, a channel in which a sealing element such as O-ring 27can be disposed. Positioned substantially centrally in and extendingthrough the wall of lower hemisphere 22 is an aperture normally closedby threaded plug 28. Po-. sitioned substantially centrally on theinterior wall of upper hemisphere 21 and extending radially inwardly isa support or post 30 having a length somewhat shorter than a fullradius. Adjacent the base of post 30 is at least one opening 32 throughthe wall of hemisphere 21, theouter periphery of opening 32 beingenclosed within sealing means such as stuffing box 34.

The compass of FIG. 1 also includes a rotor shown in the form ofcircular card 36 having attached thereto one or more bodies, such as barmagnet 38, which have associated therewith a magnetic dipole field ofsufficient strength to interact the earths magnetic field (whennon-aligned therewith) and generate a torque tending to turn the cardand magnet into alignment with at least the horizontal component of theearths magnetic field.

Card 36 is mounted for free rotation about a pair of central colinearpivots pins 40 and 41 extending perpendicularly to the plane of card 36in opposite direction to one another. Pivot pins 40 and 41 are engagedwithin corresponding pivot bearings 42 and 43 which in turn are fixed toframe 44. Card 36, shown in fragment in FIG. 2, is provided with aplurality of annular, concentric information channels or tracks ofdifferent radii such as tracks 46, 47, 48, 49, 50 and 51, it beingunderstood that the number of tracks shown is merely exemplary and notlimiting. The tracks are coded in a repetitive symmetrical code, forexample binary code, Gray or cyclic binary code, binary coded decimal orthe like. In the preferred form, coding is based upon coded modulationof radiation, as by transmission through radiation-permeable ortransparent areas of each track.

The number of concentric tracks on disk 36 is determined by the degreeof resolution desired for the compass. For example, if it is desired toobtain a resolution within 1, i.e., to divide the compass card into 360,then the individual tracks must each contribute one bit of a binarynumber at least as large as 360 in decimal notation. This serves toprovide a unique number identifying each degree. The smallest number ofbinary bits which will define any number from to 360 decimal is 2 andthus nine tracks are required for the above noted resolution. Theresolution is in theory limited only by the number of tracks and inpractice, a 3 /2 inch diameter compass card can be made with aresolution of 2 The tracks are each radially divided into equiangluaralternating opaque and transparent segments, the total number ofsegments for each track being even. Conventionally, the innermost trackrepresents the most significant binary digit, each track of greaterradius representing then a track of successively lesser significantbinary digits. Coding of disks in this manner is well known in thephotoelectric digital shaft angle encoding art, so need not be describedfurther here.

The outermost track 51 on card 36 may differ from the others in that itcan be used to provide strobe or timing information, in which case itcontains the same number of transparent segments as the leastsignificant digit track 50, preferably 360. However, while track 50 insuch case would have 360 transparent segments each of one-half degreewidth, the transparent segments of track 51 are only one-third as wideas those of track 50 and are each radially centered on a correspondingtransparent segment of track 50.

Mounted on frame 44 to one side of and preferably well spaced from disk36 is a light source such as lamp bulb 54. Also mounted on frame 44closely adjacent to disk 36 on the other side of the latter and directlyopposite to the position of bulb 54 are a plurality of photoelectricdetectors 56, 57, 58, 59, 60 and 61. Each of the latter is disposed sothat only light from bulb 54 traversing a transparent segment ofacorresponding track can be incident thereon. Thus, for example, onlylight from bulb 54 through track 51 can strike detector 56, only lightthrough track 50 can strike detector 57 and so on.

Frame 44 is supported, particulalry as shown in FIG. 3, by a gimbalstructure which comprises a U-shaped first gimbal member of yoke 64centrally locked or bolted to the end of post 30 with both arms of theyoke extending outwardly from and parallel to the long axis of post 30.A pair of pivot pins 66 and 67 are mounted on respective arms of yoke 64extending colinearly along an axis perpendicular to the long axis ofpost 30. Pins 66 and 67 are preferably so positioned that the commonaxis thereof lies along a diameter of enclosure 20.

Mounted on yoke 64 for rotation about the common axis of pins 66 and 67is a second gimbal member 68 having mounted thereon another pair ofpivots pins 70 and 71. The latter are positioned to extend colinearlyalong a common or third axis which is normally orthogonal to both thelong axis of post 30 and the common axis of pins 66 and 67. This thirdaxis also preferably lies along a diameter of enclosure 20 regardless ofthe rotational position of member 20 about pins 66 and 67. Frame 44 isfixed to bracket 72 which in turn is suspended from pins 70 and 71 sothat the axis of rotation of card 36 is also colinear with a diameter ofenclosure 20 regardless of the rotational position of the frame andenclosure with respect to one another (within, of course, the mechanicallimits or rotation of the gimbal elements).

The interior of enclosure 20 is preferably filled with a damping fluidsuch as water, a water-ethanol mixture or the like, due regard beinggiven to reserve sufficient air space 73 to provide for relief ofpressure due to thermal expansion. Electrical leads (shown only infragment in FIG. 1) to transmit signals from photocells 56 to 61inclusive and to power light source 54 are pro vided, being preferablybrought in along a common multistranded cable 76 which is preferablysealed within stuffing box 34 to prevent leakage of fluid from theinterior of enclosure 20, and thence extend into the interior of thelatter through opening 32.

In an alternative form of the compass of the present invention in placeof disk 36, there may provide a hollow tube or open-ended drum suspendedas by a spider from a spindle so as to be rotatable about thecylindrical axis of the latter. In such case the drum is provided with aseries of parallel cylindrical tracks which are coded much as tracks 46to 51 of FIG. 1 are coded.

The operation of the compass is most advantageously described asfollows: The compass is preferably filled with a damping fluid throughthe aperture provided by removing plug 28 which of course is thenreplaced. The fluid necessarily should be quite transparent to theradiation from bulb 54 and the amount of particulate matter and bubblestherein should be minimized. Because of the immersion of the gimbalstructure and card in the same damping fluid, all motions of the cardabout pivot pins 40 and 41 or about any of the gimbal pivots are dampedby the same medium. Additionally, the frictional effects operating onall of the gimbal pivot points is considerably reduced in comparison toordinary external gimbals, because the gimbals do not support a heavyfluid filled enclosure and even the few elements supported by thegimbals have a lower apparent weight because of their total immersion influid.

When the compass is energized, light from bulb 54 traverses transparentsegments of tracks 46-51 adjacent corresponding photocells 566l. Theparallel output from the photocells then constitutes a binary codedsignal which uniquely identifies the rotational position of card 36 withrespect to enclosure 20 as determined by the local magnetic field, e.g.the interaction of magnet 38 and the local horizontal component ofearth's magnetic field. As will appear hereinafter, this signal can beappropriately converted to a decimal numerical display or can be used asa control signal, for example to aid in steering a vessel.

It will be apparent that because it is not visually read directly, thecompass can be mounted a variety of locations, some quite remote fromthe user, for example at a masthead or the like. Such remote locationsat which vessel or vehicle motion such as pitching, yawing and the liketend to be exaggerated, are feasible locations for the compass of theinvention because the internal gimbal structure insures that all sixdegrees of freedom of motion of the compass card are fluid damped.

Additionally, the provision of a gimbal arrangement by which the compassis pendulously suspended for rotation in all directions about the centerof the enclosure permits one to build a compass in which the exteriordimension need be only slightly greater than the diameter of the compassrotor and allows for easy assembly of the device.

To convert the position of the compass card to a visual digital display,the present invention employs an electronic circuit shown in blockdiagram in FIG. 4. The output signals from the various photocells in thewhole compass shown at 75 are shown as being coupled along bus 76 tofirst digital comparator 78. There is also included first digitalcounter 80 connected to count the pulses in the output from digitalclock 82. The latter is connected so as to be started by a start signalon first input line 83 and stopped by a gated signal from comparator 78on output line 84. A bus 86 is provided for coupling comparator 78 tocounter 80 so that the state of the latter can be compared by comparator78 with the signals on bus 76.

The basic system thus described operates as follows:

The portion of the compass card such as disk 36 is encoded by photocellsinto a coded group of signals and the latter is fed into comparator 78on bus 76. When a start signal, which may be derived from manualoperation of a switch or automatically from a timer device, appears online 83 and starts clock 82, the latter produces a pulse train of fixedrepetition rate and counter 80 counts these pulses. "If, for example,the compass heading is 240, when the count in counter 80 reaches thenumeral 240, the output signal from comparator 78 will stop clock 82 ifappropriately gated. The count in counter 80 willbe indicative of thecompass heading.'

This foregoing sigmple device preferably includes other importantancillary systems.

Particularly, the present invention preferably includes an off-coursedetection system comprising course selection generator 90 adapted to bemanually set to provide electrical signals indicative of a desiredcourse. To this end generator 90 can include a keyboard with a pluralityof keys having decimal indicia, a power source and a diode matrix forencoding the key positions into a numerical code compatible with thenumerical system used to count in counter 80. Alternatively, generator90 can comprise a number of rotatable thumb wheels which aremultiposition switches such as those sold under the trade designationDigiswitch by the Digitram Co., Pasadena, California, to provide therequisite numerical coding of the thumb wheel positions, or can comprisea direct encoding keyboard operated switching system such as thosedisclosed in U.S. Pats. No. 3,564,541, No. 3,419,697 and the like.

The output signals from generator are fed to one side of comparator 92which is preferably a digital comparator of the same type as comparator78. Second digital counter 94 is also connected to clock 82 forconnecting the output of the latter. However, counter 94 is a known typeof up-down or reversible counter capable of counting in eitherdirection. Counter 94 is coupled to comparator 92 by bus 95 so that thecomparator can compare the state of counter 94 with the signals fromgenerator 90 to provide an output signal when the two are identical.

The output of comparator 92 is connected back to counter 94 so as toreset the latter to its zero state, and is also connected to the setinput of a bistable pulse generator or flio-flop 96. The output of thelatter is connected to an enabling input of AND gate 98, and anotherinput to the latter is line 84. The output of gate 98 is connected toclock 82 so that a signal appearing thereon will start the clock. Thereset input to flip-flop 96 is connected to line 83.

The device of FIG. 4 further includes a 180 detect circuit 100 which canbe simply a comparator having a preset number against which it comparesinput signal, in this case the preset number being the equivalent ofdecimal 180. The input to circuit 100 which provides signals to becompared is connected to bus 95. The output of circuit 100 is connectedto an up-down control input of counter 94.

Coupled through switch 101 to both counter 80 and counter 94 forpre-loading alternatively either of counters to a preset value ismagnetic variation signal generator 102 which typically is a thumbwheelswitch of the type heretofore described in connection with generator 90.Lastly, connected to bus 95 is read-out circuit 104, typically athree-decimal digit display unit having internally therein means forconverting to decimal form whatever code the system of FIG. 4 employs inthe counters, and any desired display amplifiers and drivers.

in operation, as previously noted, compass 75 provides a parallel codedoutput, preferably one which constitutes a binary coded signal uniquelyidentifying the rotational position of the compass card with respect toits enclosure. The'signal, for exemplary purposes can be assumed to bethe binary equivalent of decimal 010, representing then a heading orcourse of 010 Magnetic. It may also be assumed for purposes of theexample that either any desired course signal or magnetic variation isto be introduced into the device by respec tive generators 90 and 102.

An input pulse or start signal, applied on line 83, starts clock 82 andresets flip-flop 96 so that the output of the latter cannot enable gate98. Counter 80 and counter 94 then both start to count the output ofclock 82, both from zero and in the direction of an increasing count.When each of the counters simultaneously reaches the count of 010,comparator 92 provides a pulse which sets flip-flop 96. The output ofthe set flipflop then enables gate 98. At the same time, the count incounter 80 having matched the output from compass 75, comparator 78produces a pulse which, passing through enabled gate 98, stops clock 82.The count in couner 94 is identical, of course, to the count in counter80, and this is read out on device 104, thereby indicating the compasshead.

It can be assumed for another example that, using the same numericalvalue for the compass heading, the

thumb wheel switches or keyboard or generator 90 is set to provide asignal which is the binary equivalent of decimal 020, representing thenadesired course of 020. Lastly, it can be assumed that the output ofgenerator 102 is set to provide a'signal which is the binary equivalentof decimal O04 representing a local magnetic variation of 4 W. In thedevice of FIG. 4, switch 101 is arranged so that if the local magneticvariation is westerly, the output of generator 102 is applied to counter80; conversely if the variation is easterly, the output of generator 102is applied to counter 94. In the example given, of course switch 101 isset so that the binary signal from generator 102, the equivalent todecimal 004, is preloaded into counter 80 so that the initial state ofthe latter is then the equivalent of 004.

Again as in the previous example, an input pulse on line 83 starts clock82 and resets flip-flop 96 to disable thereby gate 98. Counter 80 thenstarts at its preloaded count and counts up to a point at which theoutput of comparator 84 is applied to gate 98 as in the previousexample. However, in this case, when counter 80 reaches the count of010, the count in counter 94 is 006. Thus, when comparator 84 produces apulse due to the matching of its inputs on lines 76 and 86, gate 98remains disabled and passes no signal. Both counters continue to countthe pulses from clock 82 so that when counter 80 now reaches the countof 024 the count in counter 94 has now reached 020. At this point,comparator 92 now produces an output signal or pulse which servesseveral functions. First, the pulse sets flipflop 96 to provide anenabling output to gate 98. Further, it resets counter 94 to its base orzero state, and lastly, it enables detector 100. Although gate 98 isenabled, there being no output at this point from comparator 84, clock82 continues to run.

The two counters then continue to count the clock output, counter 80proceeding from the count of 024 and counter 94 from its base statewhich here is 000. When counter 80 reaches a count of 204, and counter94 thus has reached a count of I80, enabled detector 100 then producesan output signal or pulse which now reverses counter 94, causing thelatter to count backward or down. The two counters continue to run nowin opposite directions until counter 80 reaches a count of 359, at whichpoint the total in counter 94 is 025, having counted down from 180.Counter 80, on the next count goes'to 000 inasmuch as the preload isapplied only to the counter at the beginning of or prior to counting,and counter 80 now begins to count up again until it reaches 010. Atthat point, comparator 78 produces an output signal which, being passedby enabled gate 98, stops clock 82. The count in counter 94, appearingon readout device 104 is then Ol4 which is the difference ordifferential between the magnetic heading indicated by compass 75, thedesired course inserted into generator 90 as corrected by the localvariation preloaded by way of generator 102.

If one wishes, the device may also include another comparator connectedfor comparing the state of reading in counter 94 with another presetnumber, thereby to produce an output when the off-course value exceedsthe preset number. This latter output can be used to trigger an alarm orthe like.

Referring now to FIG. 5 there is shown yet another variation of thedevice of the present invention particularly adapted for use inconnection with a code wheel or disc 36 which is coded in Gray code. Toconvert the Gray code to binary, outputs lines 200 to 205 inclusive fromphotocells 56 to 61 inclusive (using the same five sensor configurationwhich is exemplary of FIG. 1, but which it is understood, as indicatedby the dotted line delineating sensor 57, is a much smaller number thanwould actually be used in practice) are coupled to respective inputs ofa Gray-to-binary converter 206.

Typically, the latter is a cascaded series of gates, as is well known inthe art, and is typified by the system described in ElectronicAnalog-Digital Conversions, H. Schmid, Van Nostrand Reinhold Co., (1970)pp. 312-313.

The outputs of the converter, a group of parallel leads each bearing asignal indicative of the states of the sensors reexpressed in binarycode, is coupled to inputs to digital comparator 208. The lattertypically is formed of a plurality of exclusive OR gates each having apair of inputs terminals, one such input terminal of each gate beingconnected to the corresponding output line from converter 206. Alsoincluded is a binary counter 210, typified by the system described inElectronic Integrated Circuit and Systems, PC. Fitchen, Van NostrandReinhold Co. 1970) pp. 340342. Each bistable counter stage of counter210 is connected to a respective other or second input terminal of theOR gates in comparator 208, the lines to each such OR gate respectivelyfrom converter 206 and counter 210 are selected to carry signalsrepresenting digits of the same numerical significance, i.e., power of2. The output of the gates of comparator 208 are all connected togetherto a common lin 212 which in turn is connected to the disable or stopterminal of clock means comprising clock 214 and flip-flop 216. Theclock typically is a substantially fixed frequency pulse generator whichis designed to operate, for example, at a rate of KHz. Flip-flop 216 hasset and reset inputs. The output of the flip-flop is connected so thatwhen the flip-flop is activated by an input signal or pulse to the setinput, the output of the flip-flop starts clock 214. When the, flipflopis reset by a signal at its reset terminal (which constitutes the stopterminal of the clock means), the-flipflop output then stops clock 214.The output of clock 214 is connected to the input count terminal ofbinary counter 210. i

The version of the present invention shown in FIG.

5 also includes means for controlling the sampling rate,

and to this end there is included slow clock 218, which is alsotypically a pulse generator of fixed frequency,

but in this instance the pulse repetition rate provided by clock 218 cantypically be about 50 Hz. The output from clock 218 is connected to theset terminal of control flip-flop 216. Lastly, the embodiment of FIG. 5includes a receiver and display unit 217 which comprises a binary codeddecimal (BCD) counter 218 and a decimal display unit 220 connected tothe counter for displaying in decimal numerals the state of the BCDcounter. The input to the count terminal of the BCD counter is a linefrom fast clock 214. Counters of this type are described in ElectronicIntegrated Circuits and Systems, Supra, pp. 343-344. Typically, thedisplay unit 220 may comprise a BCD-to-decimal converter or aBCD-to-seven-segment converter, display driver amplifiers and a glowtube or segmented display, all as well-known in the art.

In operation of the device of FIG. 5 as thus described it is preferredthat the code wheel or disc 36 be coded in Grey of cyclic binary code,and that the code disc is coded to provide only a sequence of Gray codevalues corresponding to the decimal numerals from 76 to 435.

These latter values are chosen because the transition between the Graycode equivalent of decimal numeral 435 and the Gray code equivalent ofdecimal numeral 76 is merely a one-bit change. Thus, for the entire codedisk, each transition from one value to the next adjacent value will beaccomplished with but a single bit change.

The light signals produced by such a Gray-coded disk are picked up onsensors 56-61 inclusive and are converted by the Gray-to-binaryconverter 206 to provide an output which is the binary code equivalentof the Gray code input. Depending on the nature of the gates incomparator 208, these signals may then be inverted or remainnon-inverted to obtain compatibility for those comparator gates with thesignals from binary counter 210, and are then compared with the lattersignals in the gates of comparator 208.

A pulse from slow clock 218 or some other source of a start signal isapplied to the set input of flip-flop 216 and thus starts clock 214. Itwill be appreciated that clock 219 provides a pulse typically every twohundredths of a second, but the output of clock 214 provides a pulsetypically every 10 microseconds, hence between pulses from clock 219,clock 214 can provide up to 2,000 pulses. Of these, the first pulseapplied to both counter 210 and counter 218 serves to reset both totheir respective zero states which for counter 218 is true BCD zero andfor counter 210 is the binary equivalent of decimal 76 as previouslynoted. Thus counters 210 and 218 start to count up in tandem, the stateof counter 210 being examined by comparator 208 and compared with thesignals from converter 206. The state of counter 218 can be displayedcontinuously on display 220. When the state of counter 210 exactlymatches the state of the output lines from converter 206, comparator 208will provide an output signal on line 212. For example, assuming thatcomparator 208 uses exclusive OR gates, only if each and every gate hasmatched input signals (e.g. one signal up" and the other down") will anoutput signal be provided, because any unmatched gate will keep theoutput down."

The'output signal on line 2l2 then resets flip-flop 216 turning clock214 off. The count in counter 218, made during the interval required'toachieve comparison between the count in counter 210 and the output ofconverter 206, is then displayed on display 220 until the next clockpulse from clock 218 again turns on clock 214, the counters become resetand the cycle again occurs.

it will be understood that the embodiment of FIG. 5 can be furtherimplemented with means for introducing a correction for magneticvariation, an off-cause indicator and the like as in FIG. 4. However,.aparticularly important aspect of. the embodiment of FIG. 5 is that thelatter uses but a single data line between the receiver'and display unit217 and the remainder of the device. Where one wishes to mount thereceiver and display unit at a location remote from the compass andremainder of the device, the cost is conderably reduced, installationsimplified and trouble-shooting minimized by the use of such a singledata line.

Since certain changes may be made in the above apparatus withoutdeparting from the scope of the invention herein involved, it isintended that all matter contained in the above description or shown inthe accompanying drawing shall be interpreted in an illustrative and notin a limiting sense.

What is claimed:

1. A circuit for use with a compass having a digitally coded outputindicative of the compass heading, said circuit comprising incombination:

clock means for providing a train of timed signals;

first counter means for counting said train;

means for comparing the state of said first counter means with saidcoded output;

means for arresting the counting when said state of said first countermeans bears a predetermined relationship to said coded output; outputmeans for providing an output signal determined by the state of saidcounter means after arrest of said counting, said output meanscomprising a second counter means connected for counting said train, andmeans for determining the state of said second counter; said secondcounter being connected so that the count of said train therein isarrested by said means for arresting when the latter arrests thecounting by said first counter means;

means for providing a signal corresponding to an arbitrary selectedvalue,

means for comparing the state of said second counter means with saidsignal corresponding to said arbitrary selected value; and

means for resetting said second counter means to a base counting statewhen the state of said second counter means bears a desired relationshipto said selected value.

2. A circuit as defined in claim 1 including means for starting thecount of said signals in said first counter means.

3. A circuit as defined in claim.2 wherein said means for starting thecount is operated manually.

4. A circuit as defined in claim 1 wherein said means for starting thecount is a second clock providing signals at a repetition rate slowerthan the repetition rate of said train.

5. A circuit as defined in claim 1 wherein said output means comprisesmeans for displaying, in decimal form, said output signal. I

6. A circuit as defined in claim 1 wherein said means for arrestingcomprises gate means for coupling an output signal from said means forcomparing the state of said first counter means to said clock means soas to stop the latter,

a bistable device having one output thereof connected so as to enablesaid gate means, and having a pair of input terminals, one of said inputterminals being connected to said means for comparing the state of saidsecond counter means so as to be energized by the last-named means whenthe state of said second counter means bears said relationship to saidselected value such that said bistable device enables said gate means,the other input ter'- minal of said bistable device being connected to a1 1 l 2 ond counter means whenever the state of said second 9. A circuitas defined in claim 1 wherein said predecounter means reaches apredetermined value. termined value of said second counter means is the8. A circuit as defined in claim 7 wherein said means equivalent of thedecimal value 180.

for reversing is only becomes operative responsively to 10. A circuit asdefined in claim 1 including means the resetting of said second counterupon determina-' for alternatively presetting either of said countermeans tion that the state of said counter bears said desired reto anarbitrary selected value prior to counting. lationship tosaid selectedvalue.

1. A circuit for use with a compass having a digitally coded outputindicative of the compass heading, said circuit comprising incombination: clock means for providing a train of timed signals; firstcounter means for counting said train; means for comparing the state ofsaid first counter means with said coded output; means for arresting thecounting when said state of said first counter means bears apredetermined relationship to said coded output; output means forproviding an output signal determined by the state of said counter meansafter arrest of said counting, said output means comprising a secondcounter means connected for counting said train, and means fordetermining the state of said second counter; said second counter beingconnected so that the count of said train therein is arrested by saidmeans for arresting when the latter arrests the counting by said firstcounter means; means for providing a signal corresponding to anarbitrary selected value, means for comparing the state of said secondcounter means with said signal corresponding to said arbitrary selectedvalue; and means for resetting said second counter means to a basecounting state when the state of said second counter means bears adesired relationship to said selected value.
 2. A circuit as defined inclaim 1 including means for starting the count of said signals in saidfirst counter means.
 3. A circuit as defined in claim 2 wherein saidmeans for starting the count is operated manually.
 4. A circuit asdefined in claim 1 wherein said means for starting the count is a secondclock providing signals at a repetition rate slower than the repetitionrate of said train.
 5. A circuit as defined in claim 1 wherein saidoutput means comprises means for displaying, in decimal form, saidoutput signal.
 6. A circuit as defined in claim 1 wherein said means forarresting comprises gate means for coupling an output signal from saidmeans for comparing the state of said first counter means to said clockmeans so as to stop the latter, a bistable device having one outputthereof connected so as to enable said gate means, and having a pair ofinput terminals, one of said input terminals being connected to saidmeans for comparing the state of said second counter means so as to beenergized by the last-named means when the state of said second countermeans bears said relationship to said selected value such that saidbistable device enables said gate means, the other input terminal ofsaid bistable device being connected to a source of an input statesignal and to said clock means such that a start signal from said sourcestarts said clock means and causes said bistable device to disable saidgate means.
 7. A circuit as defined in claim 1 wherein said secondcounter means is a reversible counter, and including means for reversingthe direction of count in said second counter means wheNever the stateof said second counter means reaches a predetermined value.
 8. A circuitas defined in claim 7 wherein said means for reversing is only becomesoperative responsively to the resetting of said second counter upondetermination that the state of said counter bears said desiredrelationship to said selected value.
 9. A circuit as defined in claim 1wherein said predetermined value of said second counter means is theequivalent of the decimal value
 180. 10. A circuit as defined in claim 1including means for alternatively presetting either of said countermeans to an arbitrary selected value prior to counting.